This invention relates generally to polyurethane foam plastic material, and more particularly to a modified polyurethane foam capable of being embossed, welded, tear sealed, bar sealed, sealed and cut or otherwise processed by high-frequency dielectric heating techniques.
In dielectric heating, the material being treated is pressed between electrodes across which is imposed a high-frequency voltage in the range of 2-200 megahertz, to electrically stress the dielectric and thereby generate heat internally. For dielectric embossing, the surface of one of the electrodes is contoured so as to impart a predetermined design to the material engaged thereby.
It has not heretofore been feasible to dielectrically process ordinary polyurethane foam using standard dielectric heating equipment. Since the primary concern of the present invention is to alter the dielectric characteristics of polyurethane foam so that it is responsive to a high-frequency field of the type generated by standard dielectric heating equipment, a brief outline of the principles underlying the use of such equipment is in order.
When an electrically non-conductive or insulating material is subjected to a high-frequency field, the electrically charged molecules in the material tend to change position or oscillate in response to the high-frequency alternating voltage. The resultant agitation of the molecules gives rise to internal friction and heat. This internal heat is sufficient with some plastic materials to reduce the plastic to a near liquid or softened state that can result in a weld or tear when pressure is simultaneously imposed on the material by the dielectric heating electrodes.
The two properties of a dielectric material that determine how well it will retain energy in the form of heat when stressed by a high-frequency field are the dielectric constant and the power factor of the material. The dielectric constant of a material is the measure of retained energy due to molecular deformation, or the ratio of the capacitance of a material in a given electrical configuration with a vacuum as the dielectric. The power factor is the amount of leakage current that will pass through the insulating material to produce a heat loss.
The product of the dielectric constant and the power factor of a given material determine its loss factor or loss index. When an alternating voltage is applied to the dielectric, a current called the displacement current flows through it, causing energy to be stored in the dielectric. In an ideal dielectric, all of the displacement current is stored, so that an ideal dielectric makes a perfect capacitor. An ideal resistor, on the other hand, will convert all of the current passing through into heat and no charge is stored.
Synthetic plastic materials are usually classified as dielectrics but not all such materials lend themselves to dielectric heating. This capability is determined by the loss index of the plastic material which in some instances is so low as to result in virtually no internal heat when the material is exposed to a high-frequency field.
It has been demonstrated that when the loss index is 0.2 and greater, the heating response of a dielectric material in an electric field of the type established by standard high-frequency dielectric heating equipment is satisfactory or good, but when the loss index lies in a range from 0.08 to 0.2, the response is only fair.
Below this range as the loss index diminishes, the response becomes increasingly poor. Thus, in the loss index range of 0.01 to 0.08, the response is quite poor, while in the range of 0.01 or less the response runs from extremely poor to negligible.
Thus, the loss index of a dielectric material is indicative of its ability to be heated by a high-frequency electric field. With the above-described response scale in mind, we can now consider, for purposes of comparison, typical loss index values for a variety of familiar materials.
______________________________________ Typical Loss Index Values Material Loss Index Response ______________________________________ ABS polymer 0.025 poor epoxy resins 0.12 fair polyester 0.05 poor polyvinyl chloride (PVC) 0.4 good water 0.4 good polyurethane foam 0.00055 extremely poor (typical "one shot" polyether) ______________________________________
In the case of polyurethane foam, the sample from which the above loss index value was derived, had a dielectric constant of 1.1 and a power factor of 0.0005. Since the loss index is the dielectric constant multiplied by the power factor, the loss index of polyurethane is 1.1.times.0.0005 which equals 0.00055.
Because the loss index of PVC affords a good response to high-frequency dielectric heating voltages, it is widely used in the fabrication of such products as embossed automotive upholstery in which a trim material of vinyl sheeting is placed over a flexible vinyl foam plastic pad having a fabric or fiberboard backing. Upon operation of the dielectric heating press, an embossed pattern is produced in the laminated article, in which the vinyl trim material is fused to the backing through the foam plastic vinyl pad, the plastic in the pad having been melted and cured along the embossing lines.
The flexible foam pad in the embossed article provides cushioning and shock-absorbing qualities. One can, by this technique, produce seat covers, floor mats and wall panels of various kinds, and many other cushioned and embossed products having an attractive trim. In the commercial manufacture of articles of the type wherein the heat-seal seam joining the laminations also completely defines the contour of the article, it is also possible to so construct the shaped electrode as to form along the heat-seal lines, a tear-line permitting ready stripping of the waste material from the completed article. This does away with the need for a subsequent cutting operation and leaves a well-defined edge on the article.
In articles of the foregoing type, use has generally been made of PVC foam for the padding and PVC sheeting for the trim, for such combinations of vinyl foam and vinyl sheeting or film have very similar or matching dielectric characteristics which facilitate dielectric heating. However, despite the advantages of urethane, it has not been feasible to make these articles of ordinary polyurethane foam or polyurethane foam combined with a dissimilar material such as vinyl film, woven nylon fabrics and other thermoplastic materials capable of being dielectrically heated.
Among the advantages of polyurethane foam over vinyl and other commercially-available forms of foam plastics, are that polyurethane foam has markedly superior thermal and acoustical insulating properties as well as a more uniform cell structure. Moreover, not only is polyurethane foam much lighter than vinyl foam, a significant factor in handling and transportation costs, but it is a far more economical material.
Attempts have heretofore made to alter the dielectric characteristics of polyurethane foam so as to impart thereto a loss factor which lends itself to dielectric heating techniques. One approach is that set forth in the Schickendanz U.S. Pat. No. 3,061,460 which involves the post impregnation of urethane foam of the ester or ether type with a vinyl plastic to so alter the dielectric properties of the foam as to render it dielectrically heatable.
Another approach is that disclosed in applicant's prior U.S. Pat. No. 3,244,571 in which the polyurethane foam is modified by the introduction of vinyl resin. This is accomplished by including vinyl particles in the foam-forming reaction mixture. In this way, the vinyl is diffused during the foaming process throughout the fibrous structure of the foam without filling the cells thereof, so that the structure of the foam retains its normal cushioning and acoustic insulating properties that would otherwise be degraded had the cells been impregnated. Other examples of post-impregnation may be found in the Dugan U.S. Pat. No. 3,393,119, the Fishbein U.S. Pat. No. 3,535,197 and the Hand U.S. Pat. No. 3,585,062.
The present invention is concerned with improving the physical and dielectric characteristics of pre-treated polyurethane foam of the ester or ether type incorporating an additive such as PVC which is included in the urethane-foam forming reaction mixture to modify the dielectric properties of the resultant foam material so as to render it responsive to dielectric heating. The expression, "modified polyurethane foam," will hereinafter be used to designate this type of foam material to distinguish it from foam whose properties are altered by post impregnation.
The difficulty experienced with modified polyurethane foam is that the introduction of the additive in the foaming process is such as to create holes in the final product. The existence of such holes militates against the commercial acceptability of the modified polyurethane.
When a polyvinyl additive in particulate foam is intermingled with the liqud polyurethane foam-forming reaction mixture, the particles are later softened and liquified as a result of the exothermic reaction which takes place when the liquid foaming reaction mixture is laid down and foamed. But with PVC additives of the type heretofore used, the gell point of the PVC was close to the temperature of the exothermic reaction (about 300.degree. F.); hence the additive was slow to soften during foaming or failed to soften. The exothermic temperature depends on the nature of the mixture and, in practice, goes as low as 270.degree. F. and as high as 330.degree. F.
Also because the viscosity of the liquefied polyvinyl of the type heretofore used as an additive was distinctly greater than the viscosity of the urethane, extrusion of the softened additive was retarded and the polyvinyl was not adequately distributed throughout the structure of the polyurethane foam body.
Moreover, because the sizes and shapes of the PVC particles heretofore employed as an additive were such as to impair the flowability of the polyurethane-foam-forming mixture into which the particles were introduced and to render the mixture sluggish, it became difficult to pump the mixture. As a result, air was entrapped in the mixture, creating air pockets or holes in the final foam product.